Error log for filling up

Hi there,

My Error Log is filling up every couple of hours with thousands of entries that look like this:

[Tue Mar 21 08:00:20.445623 2017] [:error] [pid 28510] [client 82.71.185.245:59320] PHP Notice: Undefined index: \x90 in /var/www/MY_DOMAIN/public_html/admin/addons/apps/perch_shop/lib/vendor/symfony/polyfill-mbstring/Mbstring.php on line 179, referer: https://MY_DOMAIN/search?q=map&page=4 [Tue Mar 21 08:00:20.445625 2017] [:error] [pid 28510] [client 82.71.185.245:59320] PHP Notice: Undefined index: \x90 in /var/www/MY_DOMAIN/public_html/admin/addons/apps/perch_shop/lib/vendor/symfony/polyfill-mbstring/Mbstring.php on line 179, referer: https://MY_DOMAIN/search?q=map&page=4

Any ideas?

Perch Runway: 2.8.34, PHP: 7.0.15-0ubuntu0.16.04.4, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: b5c5906d452ec590732a93b051f3827e02749b83 $, with PDO Server OS: Linux, apache2handler Installed apps: content (2.8.34), assets (2.8.34), categories (2.8.34), collection_9 (2.8.34), swt_basket_details (2.1), perch_blog (4.6), perch_events (1.9.3), perch_forms (1.8.3), perch_gallery (2.8.6), perch_mailchimp (2.0.1), collection_6 (2.8.34), perch_shop_orders (1.0.10), perch_shop_products (1.0.10), swt_membership (2.1), perch_shop (1.0.10), collection_5 (2.8.34), perch_comments (1.2), perch_members (1.5), perch_twitter (3.5.1) App runtimes: <?php $apps_list = array( 'content', 'categories', 'geolocate', 'donations', 'perch_comments', 'perch_gallery', 'perch_forms', 'perch_members', 'perch_mailchimp', 'perch_twitter', 'perch_blog', 'perch_events', 'perch_shop', 'url_helper', 'swt_membership', 'swt_basket_details' ); PERCH_LOGINPATH: /admin PERCH_PATH: /var/www/sussexwildlifetrust.org.uk/public_html/admin PERCH_CORE: /var/www/sussexwildlifetrust.org.uk/public_html/admin/core PERCH_RESFILEPATH: /var/www/assets.sussexwildlifetrust.org.uk/public_html Image manipulation: GD PHP limits: Max upload 20M, Max POST 8M, Memory: 128M, Total max file upload: 8M F1: 6a33f95eca3667f9e0c39bf5ca2980fe Resource folder writeable: Yes HTTP_HOST: sussexwildlifetrust.org.uk DOCUMENT_ROOT: /var/www/sussexwildlifetrust.org.uk/public_html REQUEST_URI: /admin/core/settings/diagnostics/ SCRIPT_NAME: /admin/core/settings/diagnostics/index.php